My problems with Alive

I forget what volume but there was this old man that taisuke met along the way who could see the future and he told taisuke that he'd either have to pick a hard life or death. And we learn later on that the correct answer is not life, but death. However Taisuke chooses to live which is the wrong answer. But it was never properly explained how was it wrong and taisuke's life isn't really that hard at the end of the manga. Did his choice even really matter? If he had died at that moment how would it have been better?

Another question, during the same period when Hirose acquired the heart, Kutsama (or whatever his name is) said that once he gets the heart, all the power users would die - why is this? and why is it that it didn't happen...

Also, turns out that Mitama actually did something nice for humanity....but why? At the end all he did was go back to his planet. If all that needed to be done was return the heart to it's body (i'm guessing it was an alien), why go through a container (hirose) in the first place? Where is Kutsama (or whatever his name is) logic in doing this. Also why is Mitama so mean until he gets her remaining heart?

Finally, are we supposed to believe that all this time Kutsama was on mankind's side? ... wtf...where did this come from? I kind of skimmed over these pages but what were the glimpses of a pregnant woman, was that his wife? it's so unclear.

Now, I'm aware of the fact that a lot of time had passed between your post and mine, but I'll answer your question with what I noticed in this manga.

First of all, it had never been explained that it was at that time he had to make "that" choice - there were a few situations later on when he had the same choices put in front of him, and each of those could be the one of prophecy. Granted, we were supposed to believe that it was that situation. As for the reason why it would've been better for him to have died, if Taisuke had died, Hirose would stop using his powers, and the cleansing would complete, resulting in all power users (without those with heart fragments) dead, and Hirose becoming the perfect container for the heart. And that was Katsumata's original plan.The world itself might have not necessarily been better, but it was a choice between "easy death" and "hard life", and after the fight with Hirose, Taisuke's life wasn't exactly easy (you can also argue that the choice is between easy "own death" and hard "death of others", which for someone whose hole in the heart exists because of guilt, really is an unpleasant alternative).

Now, as to why had the power users not died when Hirose had acquired the heart, it's pretty simple. We had seen that while it had a pretty high propagation speed, it still required time to hit people, and Taisuke defeated Hirose pretty quickly, so it's safe to assume that it did not have enough time. And since it was only suppose to kill power users, instead of being a sure-kill (because being hit with it required a much bigger hole in heart to resist than the "suicide virus" did), most people weren't empty enough to survive it. And some might've been saved by their powers, such as super-speed or isolation.

What Mitama did didn't exactly classify as nice in our eyes, but for people in the world of "Alive", taking all the powers, which had been the cause of many deaths in the past years, away, was perceived as a nice thing. It's just that they were biased because of what had happened prior to that incident. S/he was so mean up till then because she was basically dying, and anyone who lived on a timer would get grumpy. And as to why was the container needed and what was the logic behind Katsumata's actions, we don't know anything about the heart, and it's possible that it did require a container not to do something nasty. And besides, if we assume that his original motive was making the heart impossible to obtain for Mitama, so making a container would count as stalling for time (though once again, if not for his pursuit of power and revenge, Hirose would've been a pretty good container, and with his powers he stood the greatest chance out of them all to defeat Mitama, which Katsumata thought was the best way out of this situation.

Now about Katsumata, we aren't really supposed to believe that he was on mankind's side. Indeed, if his plans were as we saw them in the manga, then indeed, he assumed that he was saving humanity, but he was always on his own side. He might've desired to save humanity, but his methods were still kind of despicable, and he's less of a saviour, and more of a tragic anti-hero. As to his motives, you can probably blame the fragment of heart he received when the whole thing had started, which both marked him as the one who was supposed to activate the heart, and gave him the powers to manipulate.